The invention relates to digital convergence or raster correction on the screen of a television receiver, particularly a projection television, and more particularly to an indicator or cursor for locating a position for manual adjustment of digital convergence.
Raster correction is generally used for the adjustment of parameters such as screen deflection, north-south of, or east-west distortions, pincusion distortions, non-linearity of deflection and other picture geometry faults in the horizontal and vertical directions. A particular field of application is convergence adjustment with a projection television receiver in which three monochromatic pictures are projected onto a screen by three picture tubes, one for each of the three primary colors. It is to be understood that as used herein a television receiver means any type of device having an electronic screen (raster) mode of picture reproduction. The device can be fed with a television broadcast signal or also purely as a monitor controlled by a RGB signal, a CSCC signal or separately with the luminance signal and color sub-carrier of any video signal source.
It is known in the art that a grid pattern of horizontal and vertical white lines is displayed on the screen forming a number of crossing points uniformly distributed over the screen. A certain crossing point of the raster design can be marked with a faded-in cursor and a particular deflection parameter of a selected point of the picture can be evaluated and a correction value for the point can be determined. The cursor fulfills two functions. The first function is the marking of the position of the portion of the picture for which the correction is required. The second function indicates whether or not correction is necessary and the extent of the correction needed.
In the prior art cross-shaped cursors are known. During convergence correction these cursors are set to a crossing point of the lines in such a manner that the cursor becomes coincident with the horizontal and vertical lines. In order that the cursor can be distinguished from the lines of the pattern it is known by JP-A 55 16 3980 to render the brightness of the cursor different from the brightness of the grid pattern. A further approach causes the cursor to become blinking. However all these solutions suffer from considerable drawbacks.
It has become evident that with a cursor with a brightness higher than the grid pattern a lack of definition can occur by defocussing; the cursor can even effloresce whereby the exact adjustment of the convergence is made difficult by the view of the cursor.
In order to avoid the above drawbacks it was proposed in U.S. Pat. No. 5,214,499 to use a cursor having a brightness inferior to the brightness of the grid pattern. This solution ensures that the presentation sharpness for the cursor on the screen is sufficient and that the adjustment of the convergence by view to the cursor can be performed with high accuracy.
In case the adjustment of convergence has to be performed automatically by means of a video camera it should be ensured that the brightness of the cursor and the brightness of the grid pattern differ by a factor of about 2. In connection with the last-mentioned solution it is necessary therefore to present the grid pattern with a very high brightness. This represents a heavy load for the high voltage supply sources for the individual picture tubes to a greater extent than during normal operation of the television device. As the adjustment of convergence is dependent also on the loading of the high voltage source it is impossible to achieve in this manner a convergence adjustment being optimal for normal operation.
When using a blinking cursor the periodic change between ON and OFF of the cursor is felt unpleasant during adjustment of convergence. With an automatic adjustment of convergence by means of a camera the camera has to be synchronized to the blinking of the cursor whereby hardware becomes more complex. A blinking cursor or a cursor with temporarily varying brightness therefore is not suitable for practical purposes.
Furthermore it is possible principally to present the cursor with a color different from the color of the grid pattern which normally is represented in pure white color. With a grid pattern in pure white color a faulty convergence adjustment is noticeable by colored edges which however can be noticed best at a pure-white cursor.
In a digital convergence system convergence values are determined for each display color and applied at an array of points located at intersections of an alignment grid superimposed, for example, on a black background. Alternatively, the background can be gray or shaded. In a manual digital convergence system a user may manually adjust deflection parameters at the grid intersections, in either or both horizontal and vertical scan directions, to superimpose individual colored lines to form a white grid on the black background. These adjustment values are then stored for each grid intersection and subsequently read out to provide a converged display image. During the convergence adjustment process the alignment grid display must be stable and free from transient geometric distortions, for example, resulting from instability of deflection fields forming the scanned raster, or raster size variation due to power supply loading effects causing the cathode ray tube (CRT) ultor or high voltage supply to vary.
In a CRT display, image brightness is directly related to the scanning beam current, where the darkest area represents the smallest current and the brightest area represents the greatest beam current. Thus it may be appreciated that the beam current varies dynamically at video frequency rates. It is well known that a scanned raster size is inversely proportional to the square root of the high voltage or EHT applied to the wall anode of the CRT. Hence, if the high voltage supply exhibits poor voltage regulation with beam current variation, the high voltage value will vary in response to the displayed image brightness, typically dropping with increasing beam current. As a result, the scanned raster size will vary, particularly in the area of any high brightness image content. It may further be appreciated that brightness modulation of the raster size is undesirable not only during normal image display, but also during convergence alignment where brightness modulation results in convergence errors.
In accordance with inventive arrangements a system for adjustment of convergence is provided enabling an optimal adjustment of convergence and also enabling the automatic implementation of adjustment of convergence. The cursor advantageously is formed by at least one irregularity of the grid pattern.
Automatic systems for picking up pictures require high differences in brightness between the individual objects for ensuring satisfactory operation. Contrary to this, however, irregularities within a pattern having no irregularities otherwise and having a brightness being variable within far limits are very easily noticeable by automatic systems for picking up pictures. Applying this recognition to the convergence correction in a television device yields the advantage that the brightness of the indispensable grid pattern can be adjusted in such a manner that the high voltage source is loaded only in such an extent which is typical for normal operation. In this way one of the important conditions for achieving optimal adjustment of convergence within normal operation mode is fulfilled.
According to one embodiment of the inventive arrangements the irregularity within the grid pattern is formed by an additionally represented element, e.g. by an additional line. The irregularity however can also be formed by omitting a portion of the grid pattern. This can be achieved for example by providing interruptions within one of the lines forming the grid pattern. In case only a single irregularity is provided within the grid pattern the position of the cursor is defined by the position of the irregularity with respect to the nearest positioned grid point. The definition of the position of the cursor can be simplified by providing not only one irregularity within the grid pattern but by providing several irregularities. The most simple determination of the position can be achieved if the considered crossing point of the grid pattern is surrounded by four irregularities. In this embodiment irregularities means that means interruptions of the grid lines or additionally displayed elements.
According to a modification of the invention any marks or elements displayed on the screen and forming the cursor are apart or separated from the cursor in such a way that they do not touch the area of the observed crossing point. Thereby the cursor does unambiguously identify the observed crossing point but advantageously does in no way affect the crossing point. Especially the cursor does not vary parameters like brightness, diameter, color or shape of the crossing point so that adjustment of convergence by viewing the crossing point is not made difficult or deteriorated by the cursor temporarily marking the crossing point.
According to another aspect of the inventive arrangements there is provided an image signal generator for an alignment grid indicating convergence of a color image display and position cursor on the alignment grid, the image signal generator comprising: a raster generator providing a scanned raster for a color image display; a grid generator for providing a grid signal corresponding to an alignment grid synchronized with the scanned raster and having intersections indicative of display locations for a manual adjustment of convergence; a cursor generator for providing a cursor signal synchronized with the grid signal and indicative of where on the alignment grid the position cursor is to be visible, the cursor signal interrupting the grid signal at the cursor position.
According to a further aspect of the invention there is provided a method for displaying a cursor on a convergence alignment grid, comprising the steps of:
displaying on a raster an alignment grid image defined by a grid signal; and, preventing propagation of the grid signal at selected positions of the alignment grid image, an absence of the alignment grid image at any one of the selected positions defining a cursor image, whereby the alignment grid image and average beam current generated during the displaying step is substantially unaffected by the cursor image.